5F2A Weirdness...Troubleshooting

I recently had a similar issue in a Princeton build that turned out to be a loose solder joint at the screen grid connection on the tube socket. The joint looked good, but once I started pulling on the wires a bit I noticed the poor connection.

I'm curious about this, too, Keith. I don't know much, but those plate voltages on the 6V6 do sound really high. What kind of 6V6 is that you're running? Is it perhaps an RCA 6V6GT gray glass? Or is it some kind of black glass? Those RCA 6V6GT gray glass tubes from the 50's can pop and arc under those kinds of voltages. @muchxs has noted this a few times on the forum. I ran one of those in a SF Champ running at about 400V on the plates when I had it, and that RCA 6V6GT gray glass popped and arced. The gray glass Sylvania 6V6GT that I ended up with in there did not. Like others, however, I've never seen a tube flash in the same way that one did in your video when the NFB was disengaged.

Vintage 5Y3 rectifier tube. Heyboer HY022772 PT. Reading @theprofessor 's build, I like the zener diode voltage reduction package. Hmm.. Order 4 diodes from Dave Hoffman, send all the way up here to me, not far from the elves?

Vintage 5Y3 rectifier tube. Heyboer HY022772 PT. Reading @theprofessor 's build, I like the zener diode voltage reduction package. Hmm.. Order 4 diodes from Dave Hoffman, send all the way up here to me, not far the elves?

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Thanks, RLee. I went back and read @tubeswell's post you pointed me to, and it's right on the money! (should I be surprised? no.)

Keith: Since you're not going to go back and change the PT, it seems the easiest way to go to use the zeners. I was really surprised at how easy the zener diode voltage reduction was. It really works! Perhaps I could have thought of a more elegant way to mount them, but they are doing their job so far. You could really shave some serious voltage off that B+ with several of these zeners in series. If I can do it, anyone can! And thanks again to @tubeswell for pointing all this out to me.

It looks like with the 8k primary, the load line crosses just barely to the low side of the knee on the datasheet. That forces a rapid shift of current to the screen as the grid voltage approaches zero relative to the cathode, and flirts with overdissipation at that high a voltage. Looks also like adding some resistance to increase screen sag is helping.

In plain English, that tube is just being asked to do too much at 400V in that circuit.

That sucks. Usually when that happens I go find it on Amazon, and order it via amazon prime, but they don't always have the exact thing I need. But I've been surprised lately by being able to find a lot of specialty electronic components on amazon.

Anything that crosses the border by way of any freight company, they get their palms well greased. Brokerage they call it. Or Import fees. Handling fees. Even Canada post and the Canadian Govt add their F-U fees. So to avoid this, I import 99% of what I buy from the USA myself. I have my parts shipped to a little border town. I literally walk across the border, sometimes with my mover's furniture dolly in tow. Pick up my packages and walk them back across the border into Canada. I do this about 10 times a year, as the border is 3 hour drive one way. However I go visit family in that direction and then I gather my parts. Works well, except when I am in a hurry for something, like these diodes.

I found an electronics supplier in Canada. I loaded up on several parts to replenish my inventory. Caps, resistors, diodes etc. They are now on the way as I ordered them here today. Maybe I'll have them by Friday.

When I needed diodes for my Twin, to replace the selenium rectifier, I got them on eBay. I got like 10 of 'em for a few bucks. I was actually surprised at how cheap and how quickly they shipped. Might be something for the future?

The diodes have arrived. I ordered 12V 5W 1N5349 and 15V 1N5352 5W options. 10 of each as I may get into this again on future builds.

Question: Can I mix the 12V and the 15V diodes together to dial down just the right amount of voltage?
12v X 6 qty = 72V reduction. The chassis is a tight space. Could I go with 4 x 15V plus 1 x 12V = 72V reduction however only using 5 diodes? Or say in another example I want to dial down 27V. Can hook up in series 1 12V and 1 15V diode?

Last question for review. I am to lift the PT centre tap wire that currently goes to ground. It's red with a yellow stripe in my case. Insert diodes in series. Then wire from
diodes to ground. Correct? I have a few little tag strips that outta do the job just fine I think. Thanks. -K